Coaxial connectors include inner and outer conductors and an insulator, or dielectric, lying between them. Such connectors typically are designated to have a predetermined characteristic impedance, which is usually 50 ohms, and sometimes 75 ohms to match the impedance of a cable and mating connector so as to minimize the standing wave ratio and consequent losses. A widely used and largely standard miniature plug coaxial connector has a front end where the inner conductor forms a socket for receiving a pin of 1 mm diameters and where the outer conductor has an outer diameter of 3.7 mm to engage the outer conductor of the mating connector. A dielectric material such as Teflon occupies substantially all of the space between the inner and outer conductors. The connector middle portion has a larger outer conductor diameter, and can have virtually any inner conductor diameter (since that inner conductor portion does not have to receive a pin) to achieve the desired impedance.
At thee front portion of the above prior standard plug connector, the inner conductor has a diameter of 1.4 mm, and the outer conductor has an inside diameter of about 3 mm, with the space between them filled with Teflon which has a dielectric constant of 2.55. The result is that the front portion of the connector has a characteristic impedance of 28 ohms. With the connector front portion having a characteristic impedance of 28 ohms, there is a serious mismatch with the characteristic impedance of the connector middle portion which has an impedance of 50 or 75 ohms. As a result, the prior connector gave rise to a considerable VSWR (voltage standing wave ratio) of about 1.13 to about 1.15, resulting in considerable losses. Although this mismatch and the resulting losses were known, no steps were taken to reduce the mismatch of characteristic impedances.
It is noted that a variety of dielectric materials are available for use in coaxial connectors, with Teflon (dielectric constant of 2.55) being the most common because of its relatively low losses especially at higher frequencies (on the order 1 GHz and higher). For example, U.S. Pat. No. 5,100,344 by Truong shows a coaxial connector plug where the front portion has an even larger inside diameter than the rear portion so mismatch would not be a problem with only a solid dielectric, although the patent describes using primarily air as the dielectric. U.S. Pat. No. 4,981,445 by Bacher et al describes a coaxial plug where the rear portion has about 50% air and 50% of a solid dielectric and the front portion is not surrounded by an outer conductor. Neither of these patents show a plug coaxial connector where there is a reduced diameter front end that results in a lower impedance than the rear portion or describes how to correct this problem.